On Tue, Jun 02, 2009 at 09:41:35AM +0200, Peter Zijlstra wrote:> On Tue, 2009-06-02 at 16:35 +0900, Paul Mundt wrote:> > > > We already do via select_clocksource(), if we are unregistering the> > current one then a new one with the flag set is selected. Before that,> > the override is likewise given preference, and we fall back on jiffies if> > there is nothing else. I suppose we could try and find the "best" one,> > but I think the override and manual clocksource selection should be fine> > for this.> > Ah, ok. So unregister calls select_clocksource again? That does leave us> a small window with jiffies, but I guess that's ok.> A synchronize_rcu() would fix that up, but I think a small window withjiffies is less painful than sorting out RCU ordering and synchronizationfor a corner case of a corner case ;-)

> > Now that you mention it though, the sched_clocksource() assignment within> > select_clocksource() happens underneath the clocksource_lock, but is not> > using rcu_assign_pointer().> > Right, that would want fixing indeed.> > > If the assignment there needs to use> > rcu_assign_pointer() then presumably all of the unlock paths that do> > select_clocksource() will have to synchronize_rcu()?> > No, you only have to do sync_rcu() when stuff that could have referenced> is going away and you cannot use call_rcu().> > So when selecting a new clocksource, you don't need synchonization> because stuff doesn't go away (I think :-)